As will be appreciated by those skilled in the printing industry, it is very important to precisely monitor and control certain of the rheological characteristics of printing inks. More specifically, and because the flow rate of printing inks is extremely complex, it is important to be able to measure properties such as viscosity. As is further understood by those skilled in the art, viscosity is defined as the ratio of shear stress "S" to shear rate "D". Shear stress "S" equals the shearing force, and shear rate "D" is the velocity gradient through the stressed liquid.
At the present time, perhaps the best known technique for determining the viscosity of printing ink is the "Falling Rod Viscometer". In fact, "The American Society For Testing And Materials" located in Philadelphia, Pa. has included a standard test for measuring the viscosity of printing inks by means of "falling rod type equipment" in its ANNUAL BOOK OF ASTM STANDARDS. The test is entitled "Standard Test Method for Viscosity of Printing Inks and Vehicles by the Falling-Rod Viscometer" and has a designation of: D 4040-89, and is incorporated herein in its entirety by reference. Typical of the equipment used for such testing is the LARAY Viscometer manufactured by the LHOMARGY Co. of Draviel, France; "Thwing-Albert Model LR" maufactured by Thwing-Albert Instrument Co., 10960 Dutton Road, Philadelphia, Pa.; and "Churchill" manufactured by Churchill Instrument Co., Ltd., Greenford, England. As the name of the test indicates, and as well known by those skilled in the printing industry, the presently available apparatus includes a stationary collar through which a rod (having a diameter only slightly less than the inside diameter of collar) is allowed to free fall. A sample of material to be tested is introduced to the small clearance between the rod and the collar such that the viscosity of the printing ink or other material being tested determines how fast the length of the rod travels through the collar due to gravity. For higher viscosity materials additional weights can be added to the top of the falling rod to increase the speed at which it falls through the collar. Thus, from the above, it will be appreciated that those factors necessary for determining viscosity can be measured and/or determined. Namely, the applied force due to the weight of the falling rod, the unit area based on the diameter of the rod and depth of the collar, and of course the velocity at which the rod falls.
Reasonably acceptable results can be obtained from this type of equipment when measuring materials of fairly high viscosity. However, unacceptable errors often occur when the equipment is used to measure materials having a viscosity below or in the range of about 10 poise. The reason for the unacceptable results is because the weight of the filling rod even without additional weights is simply so great that the rod falls too fast to obtain reliable results.
Therefore, it is an object of this invention to provide highly accurate apparatus and method to determine the viscosity of printing inks.
It is another object of this invention to provide substantially automatic apparatus and method for determining the viscosity of printing ink.
It is yet another object of the present invention to provide apparatus and method for measuring low viscosity materials below about 10 poise.